UI: Change about dialog bottom color (light theme)

This changes the bottom area color for the about dialog
for the light theme. This color makes it more consistent
for the theme.

UI/data/themes/Default.qss

@@ -133,7 +133,7 @@ QLabel#errorLabel {
 }
 
 * [themeID="aboutHLayout"] {
-	background-color: rgb(169, 169, 169);
+	background-color: #DCD9D7;
 }
 
 /* Preview background color */